1. Field of the Invention
This invention relates to a method of converting a binary signal representative of data into a ternary or higher signal. The signal thus converted is recorded as the arrangement of pits on a recording medium such as an optical disk and is used to accumulate the data.
2. Related Background Art
Optical recording mediums capable of recording data by the use of light and reading out the recorded data have, for example, a disk-like shape, a card-like shape, a tape-like shape or the like. These optical recording mediums include ones capable of recording and reproduction and ones capable of only reproduction.
The recording of data on such a medium is accomplished by stopping down a light beam modulated in accordance with a data signal into a minute spot, and scanning an information track formed on the medium by this light beam. By the application of the light beam, a row of pits representative of data is formed on the information track. Each pit differs in optical density from the other portions or is more recessed than the other portions and therefore is optically detectable. Also, the data thus recorded as a row of pits on the medium is read out by scanning the information track by a light beam having a predetermined power and detecting the reflected light thereof or the transmitted light thereof. During the recording and reproduction of such data, the spot diameter of the light beam applied onto the medium is controlled by auto-focusing (AF) control means. On the other hand, the application position of the light beam is controlled by auto-tracking (AT) control means so that the light beam may accurately scan the information track.
The signal recorded as described above usually is a binary signal. That is, the pits correspond to data "1" and the portions on which the pits are not formed correspond to data "0". To enhance the recording density of the medium in such a recording method, the dimensions of the pits and the intervals between the pits can be made small. However, the spot diameter of said light beam is naturally limited. Therefore, there have been limitations in making the dimensions of and the intervals between the pits recorded by the light beam small. This has been an obstacle to increasing the data containing capacity of the recording medium.
On the other hand, a method for solving the above-noted problem is proposed in British Patent No. 2,122,408, for example. In this method, data is converted into a ternary or higher signal and the converted signal is recorded as an arrangement of two or more kinds of pits on the medium. An example of such a method will hereinafter be described.
FIG. 1 of the accompanying drawings is a schematic diagram showing an optical data recording apparatus for recording data by the above-described method, and reproducing the recorded data. In FIG. 1, the reference numeral 25 designates an optical disk which is an optical recording medium. This optical disk 25 is clamped on a turntable 26 and rotated by a spindle motor 27. A light beam is applied to the rotated optical disk 25 by an optical head. This light beam is emitted from a semiconductor laser 14 contained in the optical head and is made into a parallel beam by a collimator lens 15. This parallel beam is transmitted through a beam splitter 16 and is condensed into a minute spot on the optical disk 25 and applied to the optical disk. The light reflected by the optical disk 25 is reflected by the beam splitter 16 and passes through a sensor lens 18 and is received by a photodetector 19.
During the recording of data, data represented by a binary signal is input to a buffer memory 20. The signal stored in the buffer memory 20 is supplied to an encoding circuit 21 at each block of a predetermined unit and is converted into a quaternary signal. The converted signal is supplied to a laser drive circuit 22. The laser drive circuit 22 drives the semiconductor laser 14 in conformity with this signal, and causes the laser 14 to emit a light beam intensity-modulated in accordance with this signal.
During the reproduction of the data, the laser drive circuit 22 causes a light beam of predetermined intensity to be emitted from the semiconductor laser. This light beam scans the row of pits recorded on the information track of the optical disk 25. The reflected light subjected to modulation by the row of pits is converted into an electrical signal by the photodetector 19. The output signal of the photodetector 19 is converted into a binary data signal by a decoding circuit 23. The data signal output from the decoding circuit 23 is once stored in a buffer memory 24 and thereafter taken out as an output signal.
The optical disk 25 has the characteristic that the optical density thereof varies as shown in FIG. 2 of the accompanying drawings in conformity with the power of the light beam applied thereto. When the power of the light beam is below the threshold value A, the optical density does not vary and no pit is formed on the disk. When the power of the light beam is between the value indicated by A and the value indicated by B, the optical density becomes greater, though non-linearly, with an increase in the power. If the power exceeds the value indicated by B, the optical density does not vary any more. That is, the area exceeding the value B is a so-called saturation area. In the aforedescribed method, the power of the light beam is varied over several stages between the value A and the value B, whereby a plurality of kinds of pits differing in optical density are formed on the medium. The level of signals reproduced from such pits becomes greater with an increase in the optical density. Accordingly, by comparing the level of the reproduced signals with several reference values, the kinds of the pits can be discriminated.
As described above, the use of a plurality of kinds of pits enables a multi-value (ternary or higher) signal to be recorded. Thereby, data can be recorded with high density on the recording medium. When, for example, N recorded states are made, high density of log.sub.2 N times can be achieved.
As the material of the medium whose optical density varies in conformity with the power of the applied light beam as described above, mention may be made, for example, of organic coloring matter dye such as cyanine or azelene of polymethylene. Also, instead of recording pits differing in optical density, pits may be formed in the form of depressions and the depth of the depressions may be varied, as proposed in the aforementioned British Patent No. 2,122,408.
FIG. 3 of the accompanying drawings illustrates an example of a method of converting a binary signal into a quaternary signal and recording it on a medium. In FIG. 3, data indicated by a is input as a binary signal indicated by b to the buffer memory 20 of FIG. 1. This binary signal is converted into a quaternary signal indicated by c, by the encoding circuit 21. By a light beam modulated in conformity with this signal c being applied to the medium, pits indicated by d are recorded on the medium. The reference numeral 31 designates pits corresponding to a first signal level, the reference numeral 32 denotes a pit corresponding to a second signal level, and the reference numeral 33 designates pits corresponding to a third signal level.
In the present example, two bits of the data correspond to one pit. That is, the pits 31 represent data "01", the pit 32 represents data "10", the pits 33 represent data "11", and the portions in which no pit is formed represent data "00". Accordingly, the binary data represented by (123033001) of the quaternary signal recorded by the pits is (011011001111000001).
The pits recorded as indicated by d in FIG. 3 are reproduced as a signal of such a waveform as shown in FIG. 4 of the accompanying drawings. It is because there is a constriction in the portion wherein the pits are in contact with each other that in the portion wherein "3" is continuous, a small depression is created in the reproduced signal. By comparing this reproduced signal with a plurality of slice levels, the value indicated by each pit can be discriminated. Also, the edge of each pit can be detected from the point of intersection between the reproduced signal and each slice level. This edge detection signal is used to produce a clock signal, and the reproduction of the data is effected in synchronism with this clock signal.
However, in the prior art method as described above, there has been the problem that if a number of pits indicative of the same value are continuous, the edges of These pits cannot be detected and the production of a clock signal becomes difficult.